The Legion of Super-Villains had been introduced in the Superman series, adult criminals corresponding to the three founding Legionnaires. Adventure 331 (April 1965) marks their first appearance in the Legion series itself.
Dynamo Boy holds try-outs for new Legionnaires, but uses his power belt to determine if they are corruptable, eliminating ones like Animal Lad, whose powers are much the same as Beast Boy. Tusker is rejected simply for being inept, but will return in the 80s.
The Legion of Super-Villains come to apply, pretending to be heroes, and we get the origins of these characters for the first time. It was always unusual that Cosmic King had powers along the lines of Element Lad, rather than Cosmic Boy, but at least we get his alchemist background.
Lightning Lord is the elder brother of Lightning Lad and Light Lass, and we get another retelling of the crash landing on Krobal and encounter with the lightning beasts, now with Mekt a part of the group. As usual, they are shown in their costumes before gaining their powers.
Saturn Queen is not given an origin, and her powers are illusion-casting in this story, though later tales would endow her with the ability to control minds. I suppose the two are mildly related.
Dynamo Lad knows they are really evil thanks to his power belt, and gives them Legion membership. The Legion of Subs try to apprehend them, but not only fail to do so, but are sent away and mocked by Dynamo Boy.
The Legion of Super-Villains turn on Dynamo Boy, tricking him into fleeing into the distant future, which he is unable to return from.
Superboy returns to Earth and challenges the villains to a showdown on a distant asteroid. They accept, but find themselves no match for Superboy, Mon-El and Element Lad.
Dynamo Boy is never seen again, but the adult Legion of Super-Villains do return in a classic tale.